The invention is directed to an electronic switch-mode power supply wherein a rectifier circuit connected to a line ac voltage is connected to an output stage for supplying a smooth dc voltage via an electronic switch.
Power packs fed from a line ac voltage for the dc supply of a load are known in numerous embodiments. An overview of circuit principles standard therefor is provided, for example, by Tietze-Schenk, "Halbleiter-Schaltungstechnik", Springer Verlag Berlin, 9.sup.th Edition 1989, Section 18.5 "Schaltnetzgerate", pp. 561 ff. Expressed in simplified fashion, such switched-mode power supplies comprise a rectifier circuit (frequently implemented as a rectifier bridge) connected to the feeding line ac voltage. The rectifier circuit supplies a pulsating dc voltage that is converted into a smoothed dc voltage in an output stage of the switched-mode power supply connected to it. As presented in greater detail in the above reference, these output stages can be rather differently designed dependent on the application, for example as a flow converter or a blocking oscillator.
The implementation of the output stage in and of itself is of only secondary significance for the initially cited electronic switched-mode power supply since it can be utilized in conjunction with a plurality of known output stages. What is thereby more critical, however, is that the smoothed output dc voltage is usually taken in the output stages at a storage capacitor having a relatively high capacitance. The necessary charging of this storage capacitor upon initialization of the switched-mode power supply causes a correspondingly high current at make. Not only the switched-mode power supply itself but also the entire supply chain at the preceding input side must be designed for this.
In this context, DE-A-34 02 222 discloses a circuit arrangement for limiting excess voltages in dc on-board networks. A controllable switch thereby has its switching path inserted into the mains line lying at low potential. A comparator with two inputs is also provided. One of these inputs is connected via a constant voltage source to the output side of this switching path, and the second of these inputs is connected to the other mains line. The comparator compares the signals supplied to it via its two inputs and controls the switch such that a device connected to this circuit arrangement is disconnected from the mains, given excess voltages. The controllable switch can also be designed as a semiconductor switch. On the basis of a capacitor lying in its drive circuit, this enables the high current surge at make to be reduced given a high capacitive load through the connected device.
Further, EP-A-0 423 885 discloses a power supply unit with a circuit for limiting the current at make. This circuit comprises a first semiconductor switch whose switching path, together with a resistor lying parallel to it, is connected into the supply line lying at low potential. An RC element as well as (in parallel thereto) the switching path of a second semiconductor switch are provided in the drive network of this first semiconductor switch. The control electrode of the second semiconductor switch is connected to the tap of a voltage divider to which a second capacitor in turn lies parallel. The switching path of a third semiconductor switch is also connected between the tap of the voltage divider and the supply line lying at low potential. This third semiconductor switch is controlled dependent on a control current supplied by a threshold circuit such that it blocks the supply voltage when a predetermined threshold is downwardly exceeded.
Specifically this last-cited, known circuit arrangement is very complicated in circuit-oriented terms--given all its functional advantages. What both known circuit arrangements also have in common is that they disconnect a connected output stage or a device supplied with the supply voltage from the dc supply with a switch coupled into the supply or mains line lying at low potential. They are switched-mode power supplies with what are referred to as "low side" switches. This supply line usually lies at ground reference potential. A different ground reference potential can thus derive given the different operating conditions at different assemblies, this having to be taken into consideration in the circuit design and definitely potentially making the implementation of the circuit more difficult.
This expense can be avoided given what are referred to as switch-mode power supplies with "high side" switches having controlled switches whose switching path is coupled into the supply line lying at high dc voltage potential.
However, P-channel types, with their known disadvantages, are usually utilized therein for the controlled switches when, of course, they are designed as semiconductor switches.